Hot-dip Galvanized Steel Sheets Research Articles

調質圧延における亜鉛めっき鋼板への表面テクスチャーの転写挙動

Surface texture plays an important role in product quality of steel sheet, which may reduce friction in sheet metal forming and improve paint appearance, especially in the automotive industry. Temper rolling is one of the key processes to determine the surface texture, because it is finally printed by appropriate work-roll surface.However, the printing behavior of surface texture from work-roll to steel sheet are not sufficiently understood, and the qualitative prediction are necessary for higher quality and adequate production operation. The requirements become larger for widely used hot-dip galvanized steel sheet, but the printing behavior has not been clarified.Therefore, the paper is concerned with the printing behaviors of roll surface textures on hot-dip galvanized steel surface in temper rolling. Firstly, the printing behavior is experimentally investigated by using different work-roll surfaces. Then, as representative parameters, average roughness, peak per inch and waviness are characterized. Also, the mutual relations among the parameters are discussed and possible method to predict the surface parameters is proposed and evaluated.

CO 2 laser welding of galvanized steel sheets using vent holes

Joining of galvanized steels is a challenging issue in the automotive industry because of the vaporization of zinc at 906 °C during fusion welding of steel (>1530 °C). In this work, hot-dip galvanized steel sheets of 0.68 mm thick (24-gage) were pre-drilled using a pulsed Nd:YAG laser to form vent holes along the weld line and then seam welded in the lap-joint configuration using a continuous wave CO 2 laser. The welds were evaluated through optical and scanning electron microscopy and tensile/hardness tests. The vent holes allowed zinc vapors to escape through the weld zone without causing expulsion of molten metal, thereby eliminating the defects such as porosity, spatter, and loss of penetration. In addition, riveting of welds occurred so long as the weld width was greater than the hole diameter that in turn provided much higher strength over the traditional “joint gap” method.

Effect of microstructure and texture on formability and mechanical properties of hot-dip galvanized steel sheets

Lead content of zinc bath has significant effects on microstructure, texture and as a result on formability and mechanical properties of zinc-coated steel. In this study, microstructures of samples made from hot-dip galvanized sheets were examined by optical and scanning electron microscopy and their chemical compositions were analyzed by energy dispersive spectroscopy (EDS). The crystallographic orientation of the coatings was determined using X-ray diffraction and texture coefficients were calculated. Mechanical properties of galvanized sheets were determined by uniaxial tensile tests and formability of the sheets was evaluated using forming limit diagrams (FLD). From the experimental results, it was concluded that by increasing the lead content of zinc bath the thickness of gamma layer, a brittle intermetallic phase, and the spangle size were increased, and, texture coefficient of basal plane component was decreased. Such variations caused the yield strength of the coated sheet to increase, but ductility was reduced. The value of FLD0s decreased due to reduction in the number of active slip planes; this in turn caused reduction of the formability. The overall evaluation of the results indicated that good formability of hot-dip galvanized steel sheets under press conditions could be attributed to the lubricating effect of zinc as the outermost layer of the coating.

Effect of deformation on the electrochemical behavior of hot-dip galvanized steel sheets

The main purpose of employing pre-coated steel sheets is to minimize corrosion of steel. However, coatings can be severely affected by forming processes. In the forming processes, due to the different modes of deformation, the strain levels are different and so can affect the properties of the coatings to a varying degree. Special attention has to be paid to the influence of deformation conditions on the performance of the coating, as regards protection against corrosion. The adhesion of the coating must remain good, and the surface should not be damaged during forming. Therefore, it is necessary to study the behavior of its corrosion resistance against the deformation. In this work, effect of strain path on the corrosion behavior of hot-dip galvanized steel sheets has been studied. Corrosion behavior of hot-dip galvanized steel sheets at various strain levels has been evaluated under four different strain paths namely, biaxial, plane strain, uniaxial (drawing) and tensile modes. The sheets were deformed by a limiting dome height test (LDH) set-up. A correlation between the degree of deformation and the loss in extent of corrosion protection offered by the coating has been established by carrying out electrochemical studies such as potentiodynamic polarization and impedance spectroscopy (EIS) in 3.5% NaCl solution. The present study shows that increase in deformation increases the extent of delamination of the coating for all the modes of deformation. The severity of deformation on delamination, however, has been found to vary in the order of, tensile < uniaxial < plane < biaxial.

Crystallographic research of spangle on hot dip galvanized steel sheets

The spangles on hot dip galvanized steel sheet were investigated on an X-ray diffraction(XRD) meter by the rotating crystal method. The correlations between the crystallographic orientations of spangles and their morphologies were analyzed. The results show that the correlation can be classified into three types: β=0°, 0°<β<90° and β=90° by β. Crystallographic model of single spangle under the ideal condition was established based on the experimental results. The correlation between α and β can be deduced by their geometric relation as an equation.

A corrosion study of hot-dip galvanized steel sheet pre-treated with γ-mercaptopropyltrimethoxysilane

In the present work an organofunctional silane, γ-mercaptopropyltrimethoxysilane (γ-MPS), has been deposited on hot-dip galvanized cold rolled steel from different silane solution concentrations. Painted and unpainted silane treated samples were corrosion tested and painted samples were adhesion tested. The surface chemistry of the unpainted silane treated samples was investigated with AES, ToF-SIMS and EDS and the surface morphology was studied with SEM. The results show that the silane film thickness is dependent on the silane concentration in the silane solution and a higher silane concentration gives a thicker film. Moreover, thicker films tend to give films with a pronounced crack pattern and even detachment of film debris. Corrosion tests of unpainted samples show that γ-MPS can not work as a passivation treatment but gives a very good adhesion to the paint.

The relationship between the fracture toughness and grain boundary characteristics in hot-dip galvanized zinc coatings

This publication presents an experimental study on the relation between the grain boundary (GB) characteristics and the intergranular cracking resistance in a hot dip zinc coating. The cracking was studied using in situ tensile tests in a scanning electron microscope on small tensile samples of a hot dip galvanized steel sheet. In situ testing offered a series of advantages like monitoring the kinematical evolution of cracking without unloading, or making micrographs and OIM imaging on the same area of the tensile sample. The grain boundaries were classified into random and special boundaries (respectively Low angle boundaries and Coincidence site lattice—CSL boundaries). These special boundaries which account for 3.5% of the whole boundaries clearly show better cracking resistance than the random boundaries. The only special boundaries which present cracking failure are in an orientation with their normal direction close to the tensile direction, i.e. submitted to a maximum effective stress. The grain boundaries characteristics are obtained from EBSD individual orientation measurements.

Premature darkening problem and its prevention in galvanized sheet surface

Abstract Studies were undertaken in detail to identify the root cause of the darkening problem in unpassivated as well as laboratory simulated passivated hot dip galvanized steel sheet having regular spangle with the help of corrosion studies,i.e., salt spray, humidity chamber, atmospheric exposure and electrochemical tests. The samples were characterized by using Scanning electron microscopy/energy dispersive X-rays analysis (SEM/EDX), Auger electron spectroscopy (AES) and also measuring the volume percentage of bright and dull spangles on galvanizes sheet by a new technique developed in our laboratory. Based on the laboratory results, a special passivating solution was developed and implemented in our continuous galvanizing line (CGL), in order to improve the darkening behaviour of galvanized sheets. The sample from CGL with special passivating solution was tested and results were further corroborated by potentiodynamic polarization and SEM/EDX studies.

Influence of Die Material upon Formability and Galling of Hot-Dip Galvanized Steel Sheets

Stretching forming test was utilized to evaluate the influence of die material upon formability and galling of hot-dip galvanized steel sheets. SS41 and SKD11 were used as the substrate of die materials which CrN, TiN and DLC (diamond like carbon) were coated on SS41 and SKD11 substrate. It was found that die with coated materials performs better than that without coated materials in terms of formability and friction. Moreover, DLC coated die material shows the best performance on formability, friction and galling comparing to other coated materials.

Formability of galvanized interstitial-free steel sheets

Hot dip galvanized steel sheets are being widely used in automotive applications for better corrosion protection. Formability of hot dip galvanized steel sheets has been one of the key issues in the automotive industry's transition to more corrosion resistant vehicle parts. In the present work, formability of hot dip galvanized interstitial-free (IF) steel sheets has been characterized. Forming limit diagrams (FLDs) and strain distribution have been evaluated experimentally by stretch-forming sheet samples over a hemispherical punch. Coefficient of friction at the punch–sheet interface during deformation has been determined. Microstructural studies at the steel–zinc interface and fractographic studies on the deformed samples have been carried out. The effect of coating thickness on formability has been analyzed by using hot dip galvanized sheets with three different coating thicknesses and a comparison has been made with an uncoated interstitial free steel sheet. The presence of brittle inter-metallic phases (which reduce formability) at zinc–steel interface layer in the coated sheets appears to have a more dominant role than the beneficial effect of reduced friction due to the presence of Zn coating at the punch–sheet interface during deformation. The combined effect of these two resulted in limit strains of the coated sheets, which are marginally lower than that of the uncoated sheet.

The role of macrostructural morphology and grain size on the corrosion resistance of Zn and Al castings

Different macrostructural morphologies and grain sizes may develop due to a wide range of the operational conditions that may exist during casting. It is well known that mechanical properties depend on solidification structures. Particularly, the literature presents relationship between the materials yield strength and grain size or ultimate tensile strength and primary and secondary dendrite arm spacings. It is also reported that the corrosion resistance has been shown to depend strongly on the structural morphology and chemical composition. The aim of this study is to investigate: (a) the influence of columnar and equiaxed structures on the corrosion resistance of aluminium and zinc as-cast samples, (b) the influence of longitudinal (columnar structure) and transversal (pseudo-equiaxed structure) sections of the same sample on the corrosion resistance for both metals and (c) the influence of equiaxed grain size of hot-dip galvanized steel sheets on the corrosion resistance. In order to obtain columnar and equiaxed structures both a vertical upward solidification apparatus and a permanent steel mold casting assembly were used. The corrosion resistance was analyzed by electrochemical impedance spectroscopy technique and Tafel extrapolation method. Corrosion tests were conducted in a 3% NaCl solution at room temperature.

Metallurgical investigations into the genesis of bare spots, exfoliation, and matte coating appearance in hot dip galvanized steel sheets

Continuously processed hot dip galvanized steel sheets that exhibited bare spots, exfoliation/peel-off, and matte coating appearance were investigated to gain an insight into the genesis of such defects. Scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM/EDS) and electron probe microanalysis (EPMA) were employed in conjunction with analysis of the galvanizing bath. Analyses revealed a bath condition sensitive to surface cleanliness of the steel substrate and showed significant levels of carbonaceous residues and rolling debris on the annealed steel substrates. This improper steel surface condition has been attributed to excess oil carry-over on the cold rolled strip surface and poor burnoff in the annealing furnace.

Performance of coated steel systems exposed to different mediaPart I. Painted galvanized steel

The performance of different pre-treated galvanized steel/primer/topcoat paint systems applied on hot-dip galvanized steel sheets has been studied. Each panel was pre-treated with 5% Fe(NO 3 ) 3 +15% H 3 PO 3 solution. After this step, the panels were coated either with acrylic (AC), alkyd (AK), vinyl (VL), epoxy base-solvent (ES) or epoxy-waterborne (EW) based corrosion-inhibiting primer using zinc molybdenum phosphate as anticorrosive pigment, and with commercially available alkyd topcoat paint. Experimental behavior of these panels under standardized salt spray chamber exposure or continuous immersion in 0.5 M NaCl, 0.5 M Na 2 SO 4 or 0.25 M NaCl +0.25 M Na 2 SO 4 solutions was evaluated through periodical visual inspection (blistering and rusting degrees) and EIS measurements (corrosion evolution). Initial (dry) and final (wet) paint adhesion was also determined. EIS data has been interpreted and discussed in terms of the time dependence of the electrical (paint coating) and electrochemical (steel substrate) parameters associated with interfacial processes describing the metal/paint system deterioration. According to the electrochemical properties, visual inspection and standardized tests results, the painting systems designated as AK, ES, EW, and VL presented the best overall anticorrosive behavior under immersion conditions, but all samples failed when exposed to the salt spray chamber. System AC behaved very well in 0.25 M NaCl+0.25 M Na 2 SO 4 but failed in the other media. Adhesion changes outside the tested areas showed that under immersion conditions there was lateral diffusion. Good correlation between standardized and electrochemical tests results was also obtained.

Performance of coated steel systems exposed to different media: Part I. Painted galvanized steel

The performance of different pre-treated galvanized steel/primer/topcoat paint systems applied on hot-dip galvanized steel sheets has been studied. Each panel was pre-treated with 5% Fe(NO 3) 3+15% H 3PO 3 solution. After this step, the panels were coated either with acrylic (AC), alkyd (AK), vinyl (VL), epoxy base-solvent (ES) or epoxy-waterborne (EW) based corrosion-inhibiting primer using zinc molybdenum phosphate as anticorrosive pigment, and with commercially available alkyd topcoat paint. Experimental behavior of these panels under standardized salt spray chamber exposure or continuous immersion in 0.5 M NaCl, 0.5 M Na 2SO 4 or 0.25 M NaCl+0.25 M Na 2SO 4 solutions was evaluated through periodical visual inspection (blistering and rusting degrees) and EIS measurements (corrosion evolution). Initial (dry) and final (wet) paint adhesion was also determined. EIS data has been interpreted and discussed in terms of the time dependence of the electrical (paint coating) and electrochemical (steel substrate) parameters associated with interfacial processes describing the metal/paint system deterioration. According to the electrochemical properties, visual inspection and standardized tests results, the painting systems designated as AK, ES, EW, and VL presented the best overall anticorrosive behavior under immersion conditions, but all samples failed when exposed to the salt spray chamber. System AC behaved very well in 0.25 M NaCl+0.25 M Na 2SO 4 but failed in the other media. Adhesion changes outside the tested areas showed that under immersion conditions there was lateral diffusion. Good correlation between standardized and electrochemical tests results was also obtained.

Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part II. Damage modes

Zinc-based coatings are widely used for protection against corrosion of steel-sheet products in the automotive industry. The objective of the present article is to investigate the deformation modes at work in three different microstructures of a thin (8 µm) zinc coating on an interstitial-free steel substrate under tension, plane-strain tension, and expansion loading. Damage mechanisms are addressed in a companion article. The plastic slip and twinning activity in the zinc grains of an untempered cold-rolled coating (labeled NSK), a tempered cold-rolled coating (labeled SK), and a recrystallized coating are compared with the response of the corresponding bulk low-alloyed zinc material. The in-plane grain size in the NSK and SK coatings ranges from 300 to 600 µm, vs about 30 µm in the recrystallized coating and bulk material. The coatings exhibit a strong crystallographic texture, with the c-axis generally normal to the sheet plane. Basal slip is shown to be the main deformation mechanism in bulk zinc and the recrystallized coating, whereas pyramidal π2 slip and mechanical twinning are found to be major modes in the NSK and SK coatings. These results, obtained from an extensive, quantitative slip-line analysis combined with electron backscattered diffraction (EBSD) measurements, are explained by the constraining effect of the substrate. This effect is successfully modeled using a simple Taylor-like polycrystalline approach. The recrystallized coating behaves much like the bulk material. The interpretation of this grain-size effect between the NSK and SK coating, on the one hand, and the recrystallized coating, on the other hand, requires a full three-dimensional finite-element analysis of the multicrystalline coating provided in this work. The simulations show that strong strain gradients can develop in the recrystallized coating from the interface to the surface, which is not the case in the NSK and SK coatings.

Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part I. Deformation modes

Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part I. Deformation modes

Spangle Formation on Hot-Dip Galvanized Steel Sheet and its Effects on Corrosion-Resistant Properties

Abstract Samples of bright and dull zinc coatings produced on a continuous galvanizing line from a zinc bath containing small concentrations of aluminum and lead was examined under a scanning electron microscope. Large grain structures termed “spangles” are produced on the galvanized coatings when lead is added to the zinc bath. In the spangle, different regions can be distinguished (viz., mirror, feathery, dimpled, orthogonal dendritic, and ridged types of surface topography). The goal of the present investigation was to identify the behavior of the spangle type and size on corrosion-resistant properties of post-treated galvanized samples. Detailed microstructure analysis of the galvanized sheet was made with the help of scanning electron microscopy/energy dispersive x-ray analysis (SEM/EDX). The salt spray test and the sulfur dioxide (SO2) test were carried out to study the corrosion resistance of the galvanized samples having different spangle appearances. These tests were further corroborated by elect...

スチールハウス実住宅における軽量形鋼の耐食性寿命予測

Atmospheric corrosion on the inhabited steel framed house was investigated with Fe-Ag galvanic type atmospheric corrosion monitoring (ACM) sensors. As the environmental conditions, temperature, humidity and currents of the ACM sensors were continuously measured in several parts of the house for five years. Corrosion rates of hot-dip galvanized steel sheet, Zn-5%Al coated steel and 55%Al-Zn coated steel for the light gauge steels were also determined on the basis of exposure tests. Service life of the galvanized steels and dew condensation in the house were discussed.Amount of deposited sea salt influences the corrosion resistance of the coated steels. Under the indoor conditions of the inhabited house, the amount of deposited sea salt was negligibly small. It is difficult to evaluate the corrosion resistance of coated steel sheets using convenient corrosion tests because of too much sea salt.

Modeling the mechanical behavior of a multicrystalline zinc coating on a hot-dip galvanized steel sheet

Numerical simulations can play a major role in the understanding of deformation mechanisms in zinc coatings of galvanized steel sheets during forming processes. A three-dimensional finite element (FE) simulation of a thin zinc coating on a galvanized steel sheet has been performed taking the multicrystalline structure of the coating into account. Experimental characterization of the gauge length of a real in situ tensile specimen reveals 34 large flat zinc grains; the grain orientations are determined using the electron back-scatter diffraction (EBSD) technique. The geometry and orientation of the grains and the plastic deformation modes specific to hexagonal close-packed (hcp) metals as plastic slip and twinning are incorporated into the modeling using a classical crystal plasticity framework. The constraint effect of the substrate is evidenced by comparing the results to the computation of a zinc layer without substrate under the same loading conditions. Attention is then focused on, respectively, the initiation of plastic activity at the grain boundaries, the multiaxial stress state of the grains, the development of a strain gradient within the thickness.

Relationship between texture and corrosion resistance in hot-dip galvanized steel sheets

Abstract The addition of lead or antimony to the zinc bath promotes both excessive growth of the zinc crystals and a dendritic solidification in the form of spangles. This phenomenon affects not only the zinc coating texture but also its surface appearance and corrosion resistance. Therefore, the effects of the zinc bath chemical composition (changed by the addition of lead or antimony) on the texture and corrosion behaviour of hot-dip galvanized steel sheets were evaluated. Standardized salt spray tests as well as exposure to a 0.5 M NaClO4 solution test were employed. The surface texture was analyzed using X-ray diffraction (XRD). In order to study the corrosion performance, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) techniques were employed. From the experimental results it was concluded that the characteristic texture of zinc coatings was not modified by an increase in the antimony content, but an increment in the lead content produced a crystallographic preferential orientation with the pyramidal family planes parallel to the coating surface. This last effect deteriorates not only the surface appearance but also the corrosion resistance of the galvanized steel sheets.